The present invention is directed to the field of security control and, in particular, to electronic biometric image capturing devices.
Biometrics is the science of biological characteristic analysis. Biometric imaging captures a measurable characteristic of a human being for identification of the particular individual (for example, a fingerprint). See, for example, Gary Roethenbaugh, Biometrics Explained, International Computer Security Association, Inc., pp. 1-34 (1998), which is incorporated by reference herein in its entirety.
Traditionally, techniques for obtaining a biometric image have included application of ink to a person""s fingertips, for instance, and rolling or simply pressing the tips of the individual""s fingers to appropriate places on a recording card. This technique can be very messy due to the application of ink, and may often result in a set of prints that are difficult to read.
Today, biometric image capturing technology includes electro-optical devices for obtaining biometric data from a biometric object, such as, a finger, a palm, etc. In such instances, the electro-optical device may be a fingerprint scanner, a palm scanner, or another type of biometric scanner. The fingerprint or palm scanners do not require the application of ink to a person""s finger or palm. Instead, fingerprint or palm scanners may include a prism located in an optical path. One facet of the prism is used as the receiving surface or platen for receiving the biometric object. For example, with an optical fingerprint scanner, a finger is placed on the platen, and the scanner captures an image of the fingerprint. The fingerprint image is comprised of light and dark areas. These areas correspond to the valleys and ridges of the fingerprint.
Electro-optical devices utilize the optical principle of total internal reflection (TIR). The rays from a light source internal to these optical scanners reach the receiving surface of the device at an incidence angle that causes all of the light rays to be reflected back into the device. This occurs when the angle of incidence is equal to or greater than the critical angle, which is defined by the ratio of the two indices of refraction of the medium inside and above the surface of the device.
In the case of a fingerprint image capturing device, a finger (or fingers) is placed on the receiving surface of the device for obtaining a fingerprint image. Moisture and/or fluids on the finger operate to alter the refraction index at the receiving surface, thereby interrupting the TIR of the prism. This interruption in the TIR causes an optical image of the fingerprint to be propagated through the receiving surface and captured by a camera internal to the device.
Although the moisture and/or fluids on the finger enable the capture of the fingerprint image, excess moisture and/or fluids from the finger are undesirable and may also alter the refraction index at the receiving surface to thereby interrupt the TIR of the prism in undesirable places on the receiving surface.
For example, under certain conditions, the air in the microscopic vicinity of the fingerprint has a very high relative humidity and can only hold a certain amount of water vapor, depending on the air temperature. The temperature at which the air can no longer suspend the water in a gaseous form is known as the dew point. When the air temperature drops below the dew point, the moisture leaves the gaseous form and becomes water. If the water contacts the surface of the prism, it will break the TIR of the prism. This interruption in the TIR causes an optical image of the water on the biometric object receiving surface (e.g., a halo that is known in the relevant art as a halo effect) to be propagated through the biometric object receiving surface and captured by a camera internal to the device. As described above, this interruption in the TIR results in an undesirable visible image of the water in the image of the biometric object.
Therefore, what is needed is an apparatus and/or method for countering the effect of moisture, fluids and/or water deposited on the surface of the prism, as a result of high humidity air in the near vicinity of a biometric object to be imaged. Such an apparatus and/or method should prevent an undesirable interruption of the TIR of the prism in electro-optical biometric image capturing devices and result in prevention of a xe2x80x9chalo effect.xe2x80x9d
The present invention addresses the above-mentioned need by providing a heater assembly to heat a platen of a biometric image capturing device above room temperature. Two methods for applying heat to the platen according to the invention are described. The first method involves using an electrically conductive transparent material to apply heat to the platen. The second method involves using resistive heating elements attached to the non-optical areas of the platen (e.g., the ends) to apply heat to the platen.
Heating the platen reduces or eliminates moisture and/or fluids on a biometric object that change the relative humidity around the area of the platen where the biometric object is placed. The reduction or elimination of excess moisture surrounding the biometric object on the platen prevents a halo effect from appearing in the biometric image.
In embodiments of the invention, the heater assembly comprises an electrically transparent conductive film which dissipates power when an electrical current is emitted through the film. At least two electrical conductors are attached to the film. Each of the conductors serves as a contact point for a connector, which transfers electrical current from a power source to each of the conductors. A temperature sensor may also be attached on or near the conductive film.
In an embodiment, the heater assembly is used to directly heat the biometric receiving surface or platen. In this embodiment, the facet of the prism for receiving the biometric object is heated to prevent formation and/or remove excess moisture on the platen, thereby preventing the halo effect. In other embodiments, an adjacent face of the prism (i.e., a facet of the prism that does not receive the biometric object) is heated to prevent formation and/or remove excess moisture on the platen, thereby preventing the halo effect.
In embodiments of the invention, electrical heating elements are attached to the platen at locations where they do not affect the image illumination or fingerprint imaging. For example, in some embodiments, the electrical heating elements are located at the ends of the prism platen.
Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention are described in detail below with reference to the accompanying drawings.